Various underdrain systems have been developed for filter systems that filter water and wastewater. The underdrain systems are a key component of a filter system as they receive liquid and/or air throughout virtually all phases of filtering including washing phases and filtration phases. In washing phases, the underdrain typically directs liquid and/or air upwardly through the filter bed to remove impurities trapped in the filter bed during a filtration phase. The liquid and/or air must be uniformly distributed over the filter bed to ensure the filter bed is properly cleaned. In an upflow filter, the underdrain, during the filtration phase, directs influent upwardly through the filter bed so that impurities may be removed therefrom. In a downflow filter, the underdrain receives the effluent and conveys it to a suitable storage location for subsequent use. Because of the key nature of underdrains to the operation of the filter system, an underdrain failure often results in shutdown of the filter system for prolonged periods.
A common type of underdrain is the “lateral” style. This type of underdrain is typically made of injection molded or extruded plastic or extruded clay. The underdrain blocks are arranged in rows termed “laterals”. The laterals are typically spaced on approximately 12 inch centers leaving an approximately 1 inch clearance between the laterals. The underdrain blocks are subject to uplift forces that tend to separate the underdrain block from the filter floor. A common source of this uplift force is due to the upward flow of water and/or air pushing against the filter floor with a force equal and opposite to the resistance of the flow across the underdrain block. There are a number of different systems used to structurally connect the underdrain block to the filter floor purportedly to resist these upward forces.
FIGS. 1 and 2 depict a typical system used to anchor underdrain laterals to the filter floor. In this system, grout 2 is placed into the space between adjacent underdrain blocks 4. The grout is designed to act in concert with the anchor assemblies 6 to secure the underdrain blocks 4 to the filter floor 8. The grout 2 and the anchor assemblies 6 essentially form a reinforced concrete beam that “keys” into lugs or ribs 12 (see FIG. 2) molded into the underdrain laterals. The anchor assembly is typically bonded into holes in the filter floor with a suitable adhesive or cast into the floor during construction. A frequently used configuration is to form a seal between the underdrain and the filter floor adjacent to the filter flume using grout in order to isolate the uplift load to the flume area 14. The isolation of the load to flume area 14 limits the requirement of the hold-down system to the area of the flume. This arrangement eliminates costly hold-downs in the remaining portions of the filter. The underdrain laterals are embedded into a layer or “bed” of grout 13 placed on the concrete floor 8 to form the seal. This “bed” of grout 13 also aids in leveling the underdrain system. Referring to FIG. 2, grout strips 17 extend between adjacent underdrain blocks 4 above the flume area 14 to support the grout above the flume area 14.
In order for the underdrain installation to be structurally sound, the grout must have sufficient strength to transfer load from the laterals to the anchors and must fully encase the anchor members and key into the lugs or ribs molded into the laterals. Low strength grout or voids around the anchors or the lugs create weak areas that can lead to structural failure. However, the space between the laterals is very narrow which limits accessibility to place the grout under and around the anchors. The strength of the grout is inversely proportional to the water/cement ratio of the grout. On the other hand, the workability of the grout is directly proportional to the water/cement ratio. Thus, these characteristics of grout are conflicting and often lead to installation errors. For example, if the grout has high strength but is too stiff to properly place into the space between the laterals, there will be voids in the grout. On the other hand, if the grout is “loose” enough to properly apply between the laterals but has too high of a water/cement ratio, the grout will not have sufficient strength to hold the laterals in place over the life of the filter system. Even if the grout has the correct combination of strength and workability, the quality of the installation is highly dependent on the skill of the installer who must be very careful to break up any air bubbles and ensure no voids are present in the grout. Also, in order to be effective, the anchors must be properly located and securely installed into the filter floor slab.
FIGS. 3 and 4 depict another system used to tie the laterals to the filter floor. In this system, angles 16 or other structural members are positioned across the uppermost surface of the laterals. The angles 16 are connected to the floor using all-threads 18 or similar structures. While this system does not rely on grout as the sole means to tie the underdrain blocks to the filter floor, it still suffers from a number of significant disadvantages. The strength of the hold-down system is dependent in part upon the section modulus of the member, the strength of the material used, and the distance between the all-thread connecting to the filter floor. In order to achieve and maintain the necessary strength, angles 16 or other structural members must be relatively large in section and must be constructed of corrosion resistant materials that adds considerable cost to the underdrain system. In addition, angles 16 or other structural members must be in intimate contact with underdrain laterals 20 to properly maintain a load path. This requirement is problematic because most underdrain laterals 20 have irregular features on their upper surface or have non-structural elements such as porous plates that prohibit intimate contact. Any gaps between the underdrain laterals and angles 16 or other structural members could allow upward movement of the underdrain lateral before the hold-down system can resist the uplift forces. This situation will result in a premature failure of the underdrain system.
Most prior art hold-down systems use rebar or other threaded rod embedded into the filter floor such that the anchoring system exerts a tensile load on the filter floor. Filter floors are commonly constructed of concrete, which has very poor tensile strength compared to its compressive and shear strength.
There are various other types of mechanical hold-down systems that do not rely on grout. Most of these systems are used in false-bottom type underdrain systems and do not readily apply to lateral type underdrain systems.